1. Field of the Invention
The present invention relates to a liquid sensor and in particular to a liquid sensor which is composed of a surface-wave component.
2. Description of Prior Art
Surface-wave components have been used for decades in electronics, e.g. as bandpass filters for the high-frequency region, and manufactured in large quantities. Depending on the choice of material used for the substrate, different wave-forms are excited. For example, for gas sensor applications, Rayleigh-wave components with a deflection perpendicular to the surface are normally used. Such components, provided with a selective layer, are employed as bio/chemosensors.
If surface-wave components are to be used as sensors for the measurement of physical liquid properties, e.g. the density, the viscosity, the shear rigidity, the conductivity, the dielectric constant, etc., Rayleigh-wave modes lead to the generation of compression waves and thus to a high energy loss. For liquid sensors, components which generate purely shear waves are used. This type of wave can be excited in various piezoelectric materials, among others in all y-rotated quartz crystal cuts.
In the y-rotated quartz crystal cuts, surface shear waves (SSWs) and bulk shear waves, in particular the so-called `surface skimming bulk waves` (SSBWs), can, however, also be excited. Surface-wave components for which these wave types can be excited are commercially available, e.g. as bandpass filters in the high-frequency region, sometimes above 1 GHZ.
When surface-shear-wave components of this type are used as sensors for the measurement of liquid properties, problems arise that play no part or only a minor part when they are used as filters. The gravest of these problems arises from the simultaneous excitation of surface shear waves (SSWs) and bulk shear waves (SSBWs) and the interference of these waves. Another problem, that can lead to considerable disturbances in the measurement effect, is the so-called `triple transit echo`. These effects, which will be explained in more detail in the subsequent description of the present patent application, give rise to problems which render these known components unsuitable for liquid sensor applications.
Surface-wave sensors of the above type are described in H. Drobe, A. Leidl, M. Rost, I. Ruge, "Acoustic sensors based on surface-localized HPSWs for measurements in liquids", in: Sensors and Actuators A, 37-38 (1993), pp. 141-148. Piezoelectric resonators on the basis of surface waves are also known from E. Benes, M. Groschl, "Piezoelektrische Resonatoren als Sensorelemente", in: e & i, 112. Jahrgang 1995, Heft 9, S. 471 bis 483.